When navigating the world of high-definition television, you’ll encounter terms like 720p and 1080i. While both represent HD video resolution formats, their underlying mechanisms and resulting visual experiences differ significantly. These differences can influence your television purchase decision and ultimately shape your viewing experience.
720p, a progressive scan format, displays 1,280 pixels horizontally and 720 pixels vertically on the screen. This arrangement produces 720 horizontal lines, or rows of pixels, which are transmitted to the television or display device sequentially. Each line is sent individually, hence the “p” designation for progressive scan. The entire image, comprising all 720 lines, is transmitted every 60th of a second, resulting in a smooth and flicker-free image. The total number of pixels displayed on a 720p screen is 921,600, slightly less than one megapixel, a standard measure used in digital cameras.
In contrast, 1080i utilizes an interlaced scanning method, represented by the “i” designation. It displays 1,920 pixels horizontally and 1,080 pixels vertically, resulting in 1,080 horizontal lines. However, instead of transmitting all lines sequentially, 1080i alternates between sending odd-numbered lines and even-numbered lines to the television. This means that only 540 lines, or half the detail, are transmitted every 60th of a second. The full 1,080 lines are transmitted every 30th of a second.
While 1080i theoretically offers more detail than 720p due to its higher line count, the interlaced nature of the signal can introduce artifacts, particularly with fast-moving objects. These artifacts may manifest as jagged edges or a slightly blurred effect. The total number of pixels in a complete 1080i signal, when both interlaced lines are combined, totals 2,073,600. However, only approximately 1,036,800 pixels are transmitted every 60th of a second.
It’s important to remember that the number of pixels displayed on a 720p or 1080i screen remains constant regardless of screen size. However, the screen size directly influences the pixel density, or the number of pixels per inch.
720p, 1080i, and Your Television
High-definition television broadcasts from local stations, cable providers, and satellite services can be either 1080i (e.g., CBS, NBC, WB) or 720p (e.g., FOX, ABC, ESPN).
Despite 720p and 1080i being the primary standards for HDTV broadcasting, it doesn’t guarantee that your HDTV screen displays these resolutions.
1080p, a progressive scan format with 1920 x 1080 lines, is not used in television broadcasting. However, it is employed by certain cable and satellite providers, internet content streaming services, and is the standard format for Blu-ray Discs.
Furthermore, many televisions marketed as 720p TVs actually have a native pixel resolution of 1366 x 768, technically classified as 768p. These TVs can still accept both 720p and 1080i signals but require scaling to match their native resolution.
LCD, OLED, Plasma, and DLP televisions (Plasma and DLP TVs have been discontinued but remain in use) cannot display native 1080i signals due to their progressive scan capabilities. When a 1080i signal is detected, these TVs must scale the image to 720p or 768p (for 720p or 768p TVs), 1080p (for 1080p TVs), or even 4K (for 4K Ultra HD TVs).
Consequently, the image quality you perceive depends on the performance of the TV’s video processor. A well-performing processor will produce smooth edges and minimize artifacts for both 720p and 1080i input sources.
A telltale sign of a processor struggling is the presence of jagged edges on objects within the image. Since the processor only needs to upscale to 1080p or downscale to 720p (or 768p), the artifacts from a 1080i signal will be more noticeable.
The processor must also perform “de-interlacing,” combining the odd and even rows of the interlaced 1080i image into a progressive scan image displayed every 60th of a second. Some processors excel at this task, while others fall short.
The Bottom Line
In essence, there is no such thing as a 1080i LCD, OLED, Plasma, or DLP TV. A flat-screen TV labeled as “1080i” can accept a 1080i signal but must scale it down to 720p for on-screen display. Conversely, a 1080p TV, advertised as Full HD, can scale any incoming 720p or 1080i signal to 1080p.
Ultimately, the visual experience on a 1080i signal, whether displayed on a 720p or 1080p TV, is influenced by various factors beyond resolution. These factors include screen refresh rate, motion processing, color processing, contrast, brightness, background video noise and artifacts, and video scaling and processing.
The availability of 1080p and 720p TVs has declined with the rise of 4K Ultra HD TVs. 720p TVs are now primarily found in screen sizes of 32 inches or smaller. You’ll increasingly find 1080p TVs in this size range, and 4K UHD TVs are becoming more affordable, leading to a decrease in 1080p TVs in screen sizes of 40 inches and larger.